Dynamic bone anchor and method of manufacturing the same

ABSTRACT

A dynamic bone anchor includes an anchor member having first and second ends and a tubular section between the ends, a longitudinal axis extending from the first to second end, an outer surface, and a bone engagement structure for engaging a bone on at least a portion of the outer surface; and a longitudinal core member having a first portion and a second portion configured to be received in the tubular section and to connected to the anchor member, with the first portion configured to be spaced apart from the anchor member and movable with respect to it. The core member is made at least partially of a nickel-titanium (Ni—Ti) based shape memory alloy such that its shape after transitioning from a martensitic to austenitic phase is configured to result in a press-fit connection between the second portion and the anchor member with the second portion in the austenitic phase.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.16/148,723, filed Oct. 1, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/831,124, filed Dec. 4, 2017, now U.S. Pat. No.10,117,695, which is a continuation of U.S. patent application Ser. No.14/098,434, filed Dec. 5, 2013, now U.S. Pat. No. 9,861,415, whichclaims priority to and the benefit of U.S. Provisional Application No.61/733,769, filed Dec. 5, 2012, in the U.S. Patent and Trademark Office,the entire content of which is incorporated herein by reference; andclaims priority from European Patent Application EP 12 195 758.3, filedDec. 5, 2012, the entire content of which is incorporated herein byreference.

BACKGROUND 1. Field

The invention relates to a dynamic bone anchor and a method ofmanufacturing a dynamic bone anchor. The dynamic bone anchor comprisesan anchor member for anchoring to a bone or a vertebra, and alongitudinal core member provided in the anchor member, a portion ofwhich is movable with respect to the anchor member. The core member ismade, at least partially, of a material comprising a nickel-titanium(Ni—Ti) based shape memory alloy having superelastic properties. Themethod of manufacturing such a dynamic bone anchor makes use of theshape memory effect of the material of the core member. The dynamic boneanchor is particularly applicable in the field of dynamic bone fixationor dynamic stabilization of the spinal column.

2. Description of the Related Art

A dynamic bone anchor is known, for example, from US 2005/0154390 A1.The shaft of the bone anchor comprises an elastic or flexible section.

A further dynamic bone fixation element is known from US 2009/0157123A1. The dynamic bone fixation element includes a bone engaging componentand a load carrier engaging component. The bone engaging componentincludes a plurality of threads for engaging a patient's bone and alumen. The load carrier engaging component includes a head portion forengaging a load carrier and a shaft portion that at least partiallyextends into the lumen. The distal end of the shaft portion is coupledto the lumen and at least a portion of an outer surface of the shaftportion is spaced away from at least a portion of an inner surface ofthe lumen via a gap so that the head portion can move with respect tothe bone engaging component. The load carrier engaging component may bemade from a high strength material, for example, a strong metal or metalalloy such as CoCrMo, CoCrMoC, CoCrNi or CoCrWNi. In a particularlypreferred embodiment, the bone engaging component is made from titaniumor a titanium alloy, while the load carrier engaging portion is madefrom cobalt chrome (CoCr).

SUMMARY

It is the object of the invention to provide an improved dynamic boneanchor that allows a head of the bone anchor to perform limited motionafter anchoring the bone anchor into a bone or a vertebrae. Further, amethod of manufacturing such a dynamic bone anchor shall be provided.

With the dynamic bone anchor, bone parts or vertebrae to be fixed orstabilized are able to carry out a controlled limited motion relative toeach other. A longitudinal core member provided in the anchor member ofthe dynamic bone anchor is preferably made of Ni—Ti based shape memoryalloy that is in the superelastic metallurgical state under theconditions in which the bone anchor is used in a patient.

Superelasticity, sometimes called pseudoelasticity, involves thecreation of stress-induced martensite which simultaneously undergoesstrain when it is formed to release the applied stress. When the appliedstress is released, the thermally unstable martensite reverts toaustenite, and the strain returns to zero. This process results in highelasticity in the material.

Due to the superelastic behavior of the core member, the degree ofpossible movement of the core member relative to the anchor member isincreased compared to materials without superelasticity. The plateau inthe stress-strain diagram of a Ni—Ti based shaped memory alloy shows asubstantially constant stress exhibited when stress-induced martensitestarts forming. This provides for an overload protection, for exampleduring the screwing in of the dynamic bone anchor. In addition, theentire dynamic bone anchor can be designed with a relatively shortlength, compared to similar bone anchors made of other materials.

The head of the bone anchor can perform rotational and/or translationalmotions with respect to a central axis of the bone anchor.

The dynamic bone anchor can be provided as a modular system, wherein acore member can be selectively combined with anchor members of differentshapes, lengths, or other properties, for example different threadtypes, barbs etc. This has the advantage that the characteristics of thecore member that substantially define the dynamic characteristics of thewhole entire dynamic bone anchor, can be utilized in differentcombinations of core members with anchor members.

By providing core members having different properties, such as theircontour, length, etc., and combining them with suitable anchor members,various dynamic properties can be achieved.

With the method of manufacturing the bone anchor that makes use of theshape memory effect of the Ni—Ti shape memory alloy of the core member,the core member can be easily connected to anchor members of differentlength or other different properties. A resulting press-fit connectionbetween the core member and the anchor member has a higher strength thanconventionally generated press-fit connections.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments by means of the accompanyingdrawings. In the drawings:

FIG. 1: shows a perspective exploded view of a dynamic bone anchoraccording to a first embodiment.

FIG. 2: shows a perspective view of the dynamic bone anchor of FIG. 1 inan assembled state.

FIG. 3: shows a cross-sectional view of the anchor member of the dynamicbone anchor according to the first embodiment, the cross-section takenin a plane containing the anchor axis.

FIG. 4: shows a side view of the core member of the dynamic bone anchoraccording to the first embodiment.

FIG. 5: shows a top view of the core member shown in FIG. 4.

FIG. 6: shows a perspective view of a head of the dynamic bone anchoraccording to the first embodiment.

FIG. 7: shows a cross-sectional view of the head shown in FIG. 6, thecross-section taken in a plane containing the anchor axis when the headis in a non-deflected state.

FIG. 8a : shows a cross-sectional view of a step of manufacturing thedynamic bone anchor according to the first embodiment, wherein the coremember is selectively combined with different anchor members.

FIG. 8b : shows a schematic view of the contour of an end portion of thecore member before it is fixed to the anchor member.

FIG. 9a : shows a cross-sectional view of the assembled dynamic boneanchor according to the first embodiment, the cross-section taken in aplane containing the anchor axis.

FIG. 9b : shows a schematic view of the end portion of the core memberafter inserting the core member into the anchor member, and heating thecore member.

FIG. 10: shows a cross-sectional view of the dynamic bone anchoraccording to the first embodiment illustrating a translational movementof the head relative to the anchor member.

FIG. 11: shows a cross-sectional view of the dynamic bone anchoraccording to the first embodiment illustrating a rotational movement ofthe head relative to the anchor member.

FIG. 12: shows an exemplary stress-strain diagram of the bone anchoraccording to one embodiment.

FIG. 13: shows a cross-sectional view of a polyaxial pedicle screw usingthe dynamic bone anchor according to the first embodiment as ananchoring element.

FIG. 14: shows a cross-sectional view of the dynamic bone anchoraccording to the first embodiment used with a bone plate to providedynamic fixation to bone parts.

FIG. 15: shows a perspective exploded view of a dynamic bone anchoraccording to a second embodiment.

FIG. 16: shows a perspective view of the dynamic bone anchor accordingto FIG. 15.

FIG. 17: shows a cross-sectional view of the dynamic bone anchoraccording to FIG. 16, the cross-section taken in a plane containing theanchor axis.

FIG. 18: shows a perspective exploded view of the dynamic bone anchoraccording to a third embodiment.

FIG. 19: shows a perspective view of the dynamic bone anchor accordingto FIG. 18 in an assembled state.

FIG. 20: shows a cross-sectional view of the dynamic bone anchoraccording to the third embodiment, the cross-section taken in a planecontaining the anchor axis.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a dynamic bone anchor according to a firstembodiment, comprises an anchor member 1 in the form of a screw member,a core member 2 and a head 3. The core member 2 can be inserted into theanchor member 1 and connected thereto, and the head 3 can be connectedto the core member 2.

As further shown in FIG. 3, the anchor member 1 comprises a first end11, an opposite second end 12, and a longitudinal axis L extendingthrough the first end 11 and the second end 12. The longitudinal axis Lforms the central axis of the bone anchor. Adjacent to the first end 11,the anchor member 1 comprises a tubular section 13 with an opening atthe first end 11. The tubular section 13 extends a distance toward thesecond end 12, and has an inner diameter di and a length adapted toaccommodate a portion of the core member 2 as described below. An endsurface 13 a of the tubular section provides a stop for the insertion ofthe core member 2. The second end 12 of the anchor member 1 is formed asa tip portion. At least a portion of an outer surface of the anchormember 1 is provided with a bone engagement structure 14 that isconfigured to engage a bone or a vertebra when the anchor member 1 isused. In the embodiment shown, the bone engagement structure 14 includesa bone thread that extends over substantially the length of the anchormember 1, but it may also only extend over a smaller portion of theouter surface of the anchor member 1.

The anchor member 1 may be made of a material that has a higher modulusof elasticity than that of the material of the core member 2, meaning,the anchor member 1 is made of a stiffer material than the core member2. In some embodiments, the anchor member 1 is made of titanium orstainless steel. The anchor member 1 can also be made of abio-compatible polymer material, where the dimensions, such as thelength and wall thickness of the anchor member are such that the anchormember does not exhibit a flexible behavior when inserted into a bone.

As shown in FIGS. 4 and 5, the core member 2 is a longitudinal membercomprising a first end 21, an opposite second end 22, and asubstantially rod-shaped central portion 23. The central portion 23 hasa circular cross-section with an outer diameter d₂ that is smaller thanthe inner diameter d₁ of the tubular section 13 of the anchor member 1.Adjacent to the first end 21, there is a first connection portion 21 a,and adjacent to the second end 22, there is a second connection portion22 a. As can be seen in particular in FIG. 5, the first and secondconnection portions 21 a, 22 a, respectively, have an outer contour of asquare with rounded edges. A distance d₃ from one flat side to anopposite flat side of the square contour is slightly greater than theouter diameter d₂ of the central portion 23 of the core member 2 suchthat the second connection portion 22 a may be connected in a press-fitmanner in the tubular section 13 of the anchor member 1, as explainedbelow. The second connection portion 22 a has a length in an axialdirection adapted to provide sufficient fixation within the tubularsection 13. The first connection portion 21 a at the first end 21connects with the head 3, and has a shape similar to that of the secondconnection portion 22 a. Between the central portion 23 and each of thefirst and second connection portions 21 a, 22 a, respectively, there isa transition portion each of 21 b, 22 b, respectively, with increasingouter diameter towards its respective connection portion 21 a, 22 a. Thetotal length of the core member 2 is such that when the core member 2 isinserted into the anchor member 1, and the second end 22 of the coremember 2 abuts against the end surface 13 a of the tubular section 13 ofthe anchor member 1, at least the first connection portion 21 a and thefirst transition portion 21 b of the core member 2 project out of theopen first end 11 of the anchor member 1.

The core member 2 is made of a material that is based on anickel-titanium based shape memory alloy, preferably from Nitinol. Thematerial exhibits superelasticity. Superelasticity is present in theaustenitic metallurgical state. In particular, superelasticity ispresent in a temperature range slightly above the stress-free martensiteto austenite transition temperature, which should be the temperaturerange of use and includes body temperature. In one embodiment, the coremember 2 is made of a nickel-titanium based shape memory alloy of theELI (extra low interstitial) type, in particular Nitinol of the ELItype. Such a material is of high purity, and in particular, comprisesless oxygen compared to other Nitinol alloys that are not of the ELItype. For example, the oxygen content may be less than 0.025 wt %,preferably equal to or less than 0.010 wt %, and more preferably equalto or less than 0.005 wt %. The material has a fatigue strength limitthat can be up to two times higher than non-ELI type shape memoryalloys.

The head 3 will be described with reference to FIGS. 6 and 7. The head 3comprises a first end 31, an opposite second end 32, and aspherical-segment shaped portion 33 adjacent to the first end 31. Anengagement portion 34 for engagement with a driver is at a free endsurface of the first end 31. Adjacent to the spherical-segment shapedportion 33, there is a cylindrical neck portion 35 with a cylindricalrecess 36 for accommodating the first connection portion 21 a of thecore member 2. The length and an inner diameter of the recess 36 is suchthat the first connection portion 21 a can be accommodated therein witha press-fit connection.

FIGS. 8a to 9b illustrate steps of manufacturing the dynamic bone anchoraccording to the first embodiment. First, the core member 2 may bepre-assembled with the head 3 by connecting the first connection portion21 a to the cylindrical recess 36 of the head 3 in a press-fit manner.The second connection portion 22 a may be previously or already shapedin its final shape, as shown in FIGS. 4 and 5. Then, at least the secondconnection portion 22 a of the core member may be cooled down below amartensite finish temperature Mf, resulting in a phase transition of thematerial from austenite into martensite.

As shown in FIG. 8a , a modular system may be provided comprising atleast two anchor members 1, 1′, that differ, for example, in their shaftlengths. The core member 2 may be selectively introduced into thetubular section 13 of one of the at least two anchor members 1, 1′.

Referring to FIG. 9a , the pre-assembled core member 2 with the head 3is introduced into the tubular section 13 of one of the anchor members1, 1′, until the second end 22 of the core member 2 abuts against theend surface 13 a of the tubular section 13. Thereby, the secondconnection portion 22 a of the core member 2 is deformed such that, forexample, the flat sides are impressed so that they have a smallerdistance from each other than in the original shape, as depicted in FIG.8b . Accordingly, the second connection portion 22 a of the core member2 can be introduced into the tubular section 13 of the anchor member 1.Due to the ability of the martensite phase to deform, the insertion canbe achieved with low force and little abrasion.

In a next step, heating the second connection portion 22 a above theaustenite finish temperature A_(f) effects a phase transition frommartensite to austenite and a change of shape of the second connectionportion 22 a back to its original shape with the rounded square contouras shown in FIG. 9b resulting from the shape memory of the material.Hence, the manufacturing process uses the shape memory behavior of thecore member 2. By means of this procedure and the square shape of thesecond connection portion 22 a, a particularly strong press-fitconnection can be achieved through a distortion-fit connection using theshape memory effect, which is stronger than a press-fit connection basedon conventional machining techniques.

It should be noted that the connection between the core member 2 and thehead 3 can also be made in the same manner described above.

Referring to FIG. 10, in the assembled state, there is a gap 37 betweenthe second end 32 of the head 3 and the first end 11 of the anchormember 1. Also, there is a gap 38 between the central portion 23 of thecore member 2 and a wall of the tubular section 13. These gaps 37, 38allow the head 3 to perform translational movement with respect to theanchor member 1 in a direction substantially perpendicular to the anchoraxis L. The extent of deflection from the central axis L of the boneanchor depends on the elasticity of the material of the core member 2and the size of the gaps 37, 38, which depend on the thickness andlength of the core member 2. A translation movement may occur when thedeflection of the core member 2 is mostly in the region of the firstconnection portion 21 a.

Referring to FIG. 11, also a rotational movement of the center point ofthe head 3 around the anchor axis L is possible. For the rotationalmovement, the deflection of the core member takes place almost over thewhole length of the central portion 23 and the first connection portion21 a. Due to the superelasticity of the material of the core member 2 adeflection of the core member 2 out of the anchor axis is possible witha core member that is shorter compared to a core member made of anothermetallic material.

In use, the dynamic bone anchor is inserted into a bone part or avertebra. Because the core member 2 is in the austenitic metallurgicalstate and in the conditions of use, the core member 2 has superelasticcharacteristics. In the stress-strain diagram of the bone anchor shownin FIG. 12, a stress-strain plateau is shown. Because of the plateau,the force acting onto the head 3 during screwing-in of the bone anchorremains constant over a certain range such that an overloading of theanchor head 3 may not occur.

In the anchored state, the head 3 is capable of performing a limitedmotion. The motion is constrained by the abutment of the core member 2against the inner surface of the tubular section 13 of the anchor member1.

A first application of the bone anchor together with a stabilizationdevice is shown in FIG. 13. The bone anchor according to the firstembodiment is coupled to a receiving part 4 to form a polyaxial boneanchor. The receiving part 4 is substantially cylindrical and comprisesa top end 41, a bottom end 42, and a coaxial bore 43 extending from thetop end 41 to a distance from the bottom end 42. The bore 43 narrowstowards the bottom end 42 and has an opening 44 near the bottom end 42.Near the opening 44, a seat 45 is provided for pivotably receiving thehead 3. A U-shaped recess extends from the top end 41 to a distance fromthe top end 41 for receiving a stabilization rod 5. The U-shaped recesscreates two free legs 46, 47, which have an internal thread 48 forcooperating with a locking member 6, such as a set screw. Furthermore, apressure member 7 is provided that exerts pressure onto the head 3 suchthat the head 3 can be locked in a certain angular position bytightening the locking member 6. The bone anchor may be used with otherdesigns of receiving parts and polyaxial bone screws. Also, the head 3of the core member 2 may be designed such that it comprises a sectionfor receiving a rod and for receiving a locking member to fix the rod,as known from other monoaxial bone screws. In use, at least twopolyaxial bone anchors may be used and connected via the rod 5. Once theanchor members 1, 1′ are inserted into the bone parts or adjacentvertebrae, the heads 3, 3′ can perform a limited motion with respect tothe anchor members 1, 1′, respectively. Once each head 3, 3′ is lockedin its respective receiving part 4, the bone anchor provides for adynamic stabilization that allows small movements of the bone parts withrespect to each other, or small movements of a motion segment of thespinal column.

A second example of application of the bone anchor according to anotherembodiment is shown in FIG. 14. In this embodiment, the dynamic boneanchors according to the first embodiment are used together with a boneplate 9 comprising holes 9 a, 9 a′ with seat portions 9 b, 9 b′ forreceiving the heads 3, 3′ of two bone anchors 1, 1′, respectively. Thetwo bone anchors 1, 1′ are inserted in adjacent bone parts 101, 101′ andthe bone plate 9 bridges at least a portion of a fracture site 102. In aspecific application, a distance between the central anchor axes C ofthe two holes 9 a, 9 a′ that accommodate the heads 3, 3′ of the boneanchors, respectively, is slightly smaller than the distance between thelongitudinal axes L of the anchor members 1, 1′. Because the coremembers 2, 2′ with the heads 3, 3′ can perform a limited motion in adirection transverse to the longitudinal axis L, the bone parts 101,101′ can be drawn together at the fracture site 102 as shown by thearrows in FIG. 14.

Referring to FIGS. 15 to 17, a second embodiment of the dynamic boneanchor is described. The dynamic bone anchor according to the secondembodiment differs from the dynamic bone anchor according to the firstembodiment in that the anchor member 1″ is formed completely as atubular member 13″. That means, the anchor member 1″ is open at thefirst end 11 and at the second end 12. The core member 2″ includes a tip24 between the second connection portion 22 a and the second end 22. Thesecond connection portion 22 a is configured to be fixed in thepreviously described manner via a distortion-fit connection using theshape memory effect to the portion adjacent to the second end 12 of theanchor member 1″. The tip 24 of the core member 2″ projects out of theopen second end 12 of the anchor 1″. The tip 24 may have a smoothsurface or may have further features, such as a self tapping structure,barbs, or a roughened surface, etc. All other parts of the dynamic boneanchor according to the second embodiment are similar to the firstembodiment and the description thereof is not repeated.

Referring to FIGS. 18 to 20, a third embodiment of the dynamic boneanchor will be described. The bone anchor according to third embodimentdiffers from the bone anchor according to the second embodiment in thatthe core member 2″′ comprises a head 3″′ similar to the head 3 of thefirst embodiment at the first end 21, wherein the head 3″′ is formedintegrally with the central portion 23 of the core member 2″′. Hence,the core member 2″′ with head 3″′ is a monolithic piece, and the entiredynamic bone anchor comprises only two pieces. The outer contour of thehead 3″′ is similar to the head 3 according to the first embodiment aspreviously described.

Further adaptations or modifications of the dynamic bone anchordescribed in the embodiments can be accomplished by one of ordinaryskill in the art without departing from the scope of the invention. Forexample, the head may have any other shape suitable for connecting it toother stabilization devices such as bone plates, receiving parts foraccommodating stabilization rods etc. The head may even be omitted if afree end of the core member is suitable for connection to anotherdevice. In such a case, the free end of the core member may comprise anengagement portion for a driver. In both cases, with or without a heador a head portion, the engagement portion of the bone anchor forengagement with a tool is at a movable end of the core member.

Any kind of tip may be provided. The tip shown in the embodiments mayeven be omitted. For example, the hollow tubular anchor member accordingto the second and third embodiments, may have prongs at the second end.

The bone engagement structure may be a bone thread of any type suitablefor engaging the bone, or may be accomplished by barbs or may even beonly a roughened surface.

The embodiments may also be combined among each other, only as anexample for such a combination, the anchor member of the firstembodiment may include a core member having an integrally formed head,as described in the third embodiment.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A dynamic bone anchor comprising: an anchor member having a firstend, a second end, and a tubular section between the first end and thesecond end having an inner surface defining a bore, the anchor memberdefining a longitudinal axis extending from the first end to the secondend, and comprising an outer surface and a bone engagement structure onat least a portion of the outer surface for engaging a bone; and a coremember at least partially insertable into the tubular section, andhaving a connection portion at or near an end of the core member that isconfigured to engage the inner surface of the tubular section tomaintain an axial position of the connection portion relative to thetubular section when the core member is inserted into the tubularsection, wherein a cross-sectional shape of the bore in a first planetransverse to the longitudinal axis is different from a cross-sectionalshape of the connection portion in a plane transverse to a central axisof the core member, and wherein a minimum width of the connectionportion measured through the central axis is greater than a width of aportion of the core member adjacent to the connection portion measuredthrough the central axis. 2-16. (canceled)
 17. The dynamic bone anchorof claim 1, wherein the cross-sectional shape of the connection portionis non-circular.
 18. The dynamic bone anchor of claim 1, wherein whenthe core member is not inserted into the tubular section, a width of theconnection portion is configured to be greater than a width of the bore.19. The dynamic bone anchor of claim 1, wherein the connection portionis configured to be deformed when inserted into the tubular section. 20.The dynamic bone anchor of claim 19, wherein at least the connectionportion of the core member comprises a first material comprising anickel-titanium (Ni—Ti) based shape memory alloy.
 21. The dynamic boneanchor of claim 20, wherein the Ni—Ti based shape memory alloy isNitinol of the Extra Low Interstitial (ELI) type.
 22. The dynamic boneanchor of claim 20, wherein the anchor member comprises a secondmaterial different from the first material.
 23. The dynamic bone anchorof claim 22, wherein the second material has a greater modulus ofelasticity than that of the first material.
 24. The dynamic bone anchorof claim 1, wherein the core member forms a distal tip of the boneanchor.
 25. The dynamic bone anchor of claim 24, wherein at least thetip comprises a Nitinol of the ELI type to enhance fatigue strength. 26.The dynamic bone anchor of claim 24, wherein the tip formed by the coremember is configured to protrude from the second end of the anchormember when the core member is fully inserted into the tubular sectionfrom the first end of the anchor member.
 27. A dynamic bone anchorcomprising: an anchor member having a first end, a second end, and atubular section between the first end and the second end and having aninner surface defining a bore, the anchor member defining a longitudinalaxis extending from the first end to the second end, and comprising anouter surface and a bone engagement structure on at least a portion ofthe outer surface for engaging a bone; and a core member at leastpartially insertable into the tubular section, and having a connectionportion at or near an end of the core member that is configured toengage the inner surface of the tubular section to maintain an axialposition of the connection portion relative to the tubular section whenthe core member is inserted into the tubular section, wherein across-sectional shape of the connection portion in a plane transverse toa central axis of the core member is non-circular, and wherein a minimumwidth of the connection portion measured through the central axis isgreater than a width of a portion of the core member adjacent to theconnection portion measured through the central axis.
 28. The dynamicbone anchor of claim 27, wherein a cross-sectional shape of the bore iscircular.
 29. The dynamic bone anchor of claim 27, wherein when the coremember is not inserted into the tubular section, a width of theconnection portion is configured to be greater than a width of the bore.30. The dynamic bone anchor of claim 29, wherein the connection portionis configured to be deformed when inserted into the tubular section. 31.The dynamic bone anchor of claim 30, wherein when the connection portionis inside the tubular section, the connection portion is configured toexpand to apply an outward force on the inner surface of the tubularsection.
 32. The dynamic bone anchor of claim 27, wherein at least theconnection portion of the core member comprises Nitinol.
 33. The dynamicbone anchor of claim 32, wherein the Nitinol is of the ELI type.
 34. Thedynamic bone anchor of claim 27, wherein the core member forms a distaltip of the bone anchor.
 35. The dynamic bone anchor of claim 34, whereinat least the tip comprises a Nitinol of the ELI type to enhance fatiguestrength.